Telephone hybrids are often used with the public switched telephone network (PSTN) when the interface between a two-wire and four-wire circuit is implemented. As well known, a two-wire circuit has speech communications existing on the same wire pair, while overall switching and transmission functions are provided by a four-wire circuit with two separate sides of the four-wire circuit corresponding to transmit and receive. In those digital systems implemented in modern communication systems, each communication direction as transmit and receive is processed independently. A typical analog line card in a telephone central office includes a hybrid that adapts the four-wire network to the two-wire analog circuit. Energy passing from the four-wire to the two-wire circuit often is reflected back into the four-wire circuit section, creating an echo. This echo occurs in Voice-over-Internet Protocol (VoIP) communications systems and similar packet services such as using Session Initiation Protocol (SIP). The packet voice network typically does not require use of an echo canceller, but when it is interfaced to an analog communications line, the echo canceller is required.
Echo cancellation is important especially when an Integrated Access Device (IAD) or Integrated Communications Platform (ICP) includes Foreign Exchange Office (FXO) ports connected to the PSTN for implemented packet voice communications such as Voice-over-Internet Protocol (VoIP). When a packet voice network is connected to the PTSN using, for example, an ICP, via a two-wire analog interface, the ability of the echo canceller to operate on PSTN echo can be severely compromised if there is a strong local echo from the two-wire interface itself. Echo reduction requires that the FXO hybrid balance impedance be matched to the phone line. Because few installations are similar to each other, the amount of echo varies widely with each installation.
In some prior art systems, the impedance is adjusted by manually using one or more test calls and evaluating the echo for each impedance setting. The interface typically can have different balance impedances and the proper impedance selection can reduce echo. These prior art manual procedures were time consuming, took several minutes per line, and had to be repeated for each line. This manual process can be difficult when an Integrated Communications Platform (or similar platform) has up to ten FXO ports. For example, the manual impedance balancing method also requires on-site assistance where a test call has to be placed from a packet voice station behind the Integrated Communications Platform/Integrated Access Device (ICP/IAD) to the PSTN user. This is less than satisfactory because it is not always easy to differentiate local echo from the echo arising at some other point in the communications path. In a manual test procedure with the FXO under test, the transmit and receive gains are set to zero. The impedance is set to its default value, which is commonly about 600 ohms. Arrangements are made for someone on-site to place several test calls, while an external party receives these test calls. The test call is established from a Session Initiation Protocol (SIP) station behind the ICP to an external party through the FXO interface.
If the FXO is in a hunt group, the person performing the test uses a debug command at the ICP/IAD administrative operator console to verify that the call is going out through the right port. Otherwise, the person “hangs up” and calls again until the hunt group hits (transfers to) the desired port. After the called party answers, the debug command is used to find which DSP (digital signal processor) slot/port/channel is being used. The debug command is used to start a DSP white noise generator and echo return loss (ERL) measured on that channel. The ERL reading stabilizes in about five to about ten seconds. A human user maintains a record of the echo return loss. The impedance is set to the next value and the previous steps repeated.
After stepping through all the impedance values (for example, there could be 11), the DSP debug process is disabled and the FXO set for the impedance that yields the highest echo return loss. The transmit and receive gains are restored and the configuration saved. Optionally, it is possible to use the test call to check for echo and this is repeated for the next FXO interface.
Besides the initial drawbacks indicated above, there are other drawbacks in this manual procedure. For example, an on-site operator is required to place the test call and an off-site operator must receive the test call. Also, a user typically rotates through a hunt group to hit the desired FXO interface. It is also necessary for a user to interpret one debug output before the next one can be entered. Debug commands require up to 40 characters, which must be precisely entered. The customer configuration for transmit and receive gain then must be changed and restored.
One known utility program termed FXOtune tunes various settings on FXO modules for analog cards, optimizes the line characteristics of a time division multiplexing (TDM) device to minimize echo by balancing the hybrid, and adjusts line impedance. Another system is disclosed in U.S. Pat. No. 7,215,762, which discloses a technique that selects impedance settings for a loop start trunk line in a network that extends a four-wire circuit to a subscriber's premise, i.e., a private branch exchange (PBX), from a local exchange or central office (CO). The disclosed method selects an optimum impedance for a loop start trunk line by successively applying respective ones of a plurality of impedance settings to the loop start trunk line and for each impedance setting measuring the impulse response of the trans-hybrid echo path by applying audio test signals to obtain a time-domain trans-hybrid transfer function. It derives a frequency-domain spectrum representative of the trans-hybrid loss. It then calculates a figure of merit of the transfer function based on the power spectrum. Once all impedance settings have been tested, the optimum impedance is selected as the impedance setting associated with the highest calculated figure of merit. It uses a separate test circuit and a pre-prepared test file (215). It requires synchronization of sent and receive signals (235), and compares and analyzes the received data after the test call is dropped rather than during the test call itself.
It is desirable that a new system and method be implemented that overcomes the drawbacks identified above and can be suitable for remote operation and used without manually reconfiguring any ports under test and can initiate test calls automatically.